So I think the largest problem with understanding evolution for people is that they put the egg before the chicken (so to speak). I’m discussed evolution before but I wanted to do it in a way that makes it fairly obvious to anyone and everyone.
When you think about various changes in animals such as flippers, eyes, mouths, or anything in between you might wonder just how those came to be? We’ll start with the earliest of organisms (that I’m aware of), or perhaps the simplest, maybe both. Bacteria reproduce very quickly and this is why they are an ideal candidate for examination.
Each time an organism reproduces there are some errors, even as you age there are errors in your own bodies processes. These errors are what end up killing you (some faster than others), but occasionally the errors can work in an organisms favor. Each new generation will have a large quantity of variable mutations that will either help, hinder, or have no appreciable impact.
If the new mutation raises the odds that the organism will reproduce then it will passed on. Even better, if the mutation is so superior that it causes the new mutation to be bread at a faster pace than other mutations it will eventually dominate that species in the region and become a new “species”.
Wikipedia defines Evolutionary (or Selection) Pressure as:
Any cause that reduces reproductive success in a proportion of a population, potentially exerts evolutionary pressure or selection pressure.
This is important, any mutation that would kill or sterilize an organism before it can reproduce will not be passed on.
That’s it, that’s all evolution is, improvements in an organism because of the necessity for improvements to be improvements. Sounds convoluted but that’s really what it comes down to. Why do animals have ears? Because the rudimentary structures that lead up to ears were always better than not having them. Why have multiple species evolved eyes independent of one another? Because eyeballs are so beneficial in nearly any situation that anything that has them (or their rudimentary ancestors) was at a much greater advantage than anything that did not.
Our hearts, lungs, brains, tongues, and everything else was at one time much simpler but still fundamentally better than other mutations.
If you want to see this in action look at bacteria (once again). I am currently taking an antibiotic (I finish tomorrow), but I create an interesting ecosystem when I take it. Each new generation of bacteria will have slight differences. If any of those differences allow the bacteria that has it to survive long enough to reproduce then I will have a bacteria that is now resistant to antibiotics.
This is sensible and rather obvious when you think about it. Insects are becoming resistant to pesticides because their constant use means that the only insects left alive are those that are immune to the chemicals. It could be argued that these insects would exist regardless, and perhaps our plight is just not understanding that those remaining “super bugs” are merely the same problem without a simple solution.
This is the essence of the line “Survival of the Fittest”. People often mistake this as strength in terms of literal physical hulk strength but what is actually meant is the ability to remain fertile and viable in an environment (most importantly the environment your species resides in). Those that are most accepting of environmental shifts are the fittest.
Extinction events, by contrast, are simply events where the environment changes so dramatically that none of the mutations created are “good enough” to survive to reproduction. You can’t expect an organism to go from no eyes to eyes in a single generation and this is why dramatic environment shifts are not a good thing. We don’t want to quickly change the world’s climates because organisms will not adapt in time to keep the food chain together.
So I hope this helped. To summarize again, when a mutation in an organism provides a higher rate of reproduction it will become more prevalent in a species. If it is so much better that it dwarfs all other variants it will become the new “normal” for the species and likely considered a unique organism. Any mutations that have negative impacts on reproduction rates will likely be removed from “the system” by design (you can’t pass on genes if you don’t reproduce). Mutations that neither hinder nor help will be passed on in random amounts.